$400 million in damage to the Navy's fast attack submarine USS Miami was caused by a fire started by a vacuum cleaner.

That's what a preliminary report about the May 23 fire has found. The vacuum cleaner, the Navy said in a statement, was "used to clean worksites at end of shifts, and stored in an unoccupied space."

There's still no indication how the vacuum cleaner caught fire to begin with.

"The fire impacted the forward compartment of the submarine which includes crew living, command and control spaces and torpedo room," the Navy said. "Miami's nuclear propulsion spaces were not affected by the fire."

At the time of the fire, the submarine was at the Portsmouth Naval Shipyard in Maine undergoing an overhaul. While the submarine's nuclear plant was never in danger, it had been shut down for more than two months.

The Navy added that the incident will also cost taxpayers about $40 million more for "the secondary effects such as disruption to other planned work across all Naval Shipyards, and the potential need to contract work to the private sector."

"The Navy is conducting formal Judge Advocate General Manual (JAGMAN) and safety investigations to address lessons learned," the Navy said.

h/t: NPR's Rob Schaefer

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